Method of manufacturing a thermal conductive circuit board with grounding pattern connected to a heat sink

ABSTRACT

A sheet-like thermally conductive resin composition containing 70 to 95 wt. % inorganic filler and 5 to 30 wt. % thermosetting resin composition, a lead frame as a wiring pattern, and an electrically conductive heat sink with a metal pole placed therein are superposed, heated and compressed, and thus are combined to form one body. Consequently, a thermally conductive circuit board with a flat surface is obtained in which a grounding pattern is grounded to the heat sink inside the insulating layer. Thus, the grounding pattern and the heat sink can be connected electrically with each other in an arbitrary position inside the insulating layer of the thermally conductive circuit board. Accordingly, there are provided a thermally conductive circuit board with high heat dissipation, high conductivity and high ground-connection reliability, a method of manufacturing the same, and a power module allowing its size to be reduced and its density to be increased.

This application is a division of application Ser. No. 10/309,707, filedDec. 3, 2002 now U.S. Pat. No. 6,860,004, which is a division ofapplication Ser. No. 09/834,710, filed Apr. 13, 2001 now U.S. Pat. No.6,522,555, which applications are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to a circuit board on whichvarious semiconductor devices or electronic components are to bemounted. Particularly, the present invention relates to a resin board (athermally conductive board) with high heat dissipation that is suitablefor use in the power electronics field and to a power module includingthe same.

2. Related Background Art

Recently, there have been demands for increases in density andimprovements in function of semiconductors, as a higher performance anda smaller size are required in electronic equipment. Accordingly,smaller higher-density circuit boards on which such semiconductors areto be mounted also have been required. Hence, a design withconsideration to heat dissipation of a circuit board has becomeimportant. As a technique of improving heat dissipation of a circuitboard, a method employing an insulated metal substrate using a metalplate made of, for example, copper or aluminum instead of a conventionalprinted board formed of glass-epoxy resin has been known in which acircuit pattern is formed on one surface or both surfaces of the metalplate with an insulating layer interposed therebetween. When higherthermal conductivity is required, a board has been used in which acopper sheet is bonded directly to a ceramic board made of, for example,alumina or aluminum nitride. Generally, an insulated metal substrate isutilized for use with a relatively low power consumption. In this case,the insulated metal substrate is required to have a thin insulatinglayer to have an improved thermal conductivity. Accordingly, theinsulated metal substrate has a problem in that a portion between thecircuit pattern and the metal plate as a ground tends to be affected bynoise easily, and also has a problem in withstand voltage.

In order to solve such problems, recently, a board has been proposed inwhich a composition containing a filler with high thermal conductivityadded to a resin is combined with a lead frame as an electrode to formone body. For example, JP 10(1998)-173097 A proposes a board includingsuch a composition. A method of manufacturing such a thermallyconductive board is shown in FIGS. 15A and 15B. According to JP10(1998)-173097 A, a mixed slurry containing at least an inorganicfiller and thermosetting resin is used to form a film and thus asheet-like thermally conductive resin composition 16 is produced. Thecomposition 16 is dried and then is superposed with a lead frame 11 anda heat sink 13 as shown in FIG. 15A. Next, as shown in FIG. 15B, theyare heated and compressed and the composition 16 is cured to form aninsulating layer 12. Thus, a thermally conductive board 28 is produced.

In such a thermally conductive board and a power module including thesame, generally a part of the wiring pattern is grounded to the heatsink according to electrical requirements such as reducing straycapacitance. In such a case, it is necessary to connect the wiringpattern and the heat sink outside the board and an additional step otherthan steps for component mounting is required. FIG. 16 shows an exampleof the connection with the heat sink in such a case. According to thisexample, a part of the lead frame 11 is used as a grounding pattern 15and a lead 29 is soldered to the grounding pattern 15 and the heat sink13 to establish a ground connection. In the figure, numeral 12 indicatesan insulating layer. In order to connect the wiring pattern and the heatsink outside the board, it also is necessary to place the groundingpattern at the periphery of the board. This is a constraint in patterndesign, and the degree of freedom in circuit design has been decreasedaccordingly. Furthermore, there has been a problem in that the size ofthe board itself increases to allow the establishment of the connection.

SUMMARY OF THE INVENTION

The present invention is intended to solve the aforementioned problemsin the conventional technique. It is an object of the present inventionto provide a thermally conductive circuit board (hereinafter alsoreferred to as “thermally conductive board”) with high heat dissipation,high conductivity, and high ground-connection reliability that areobtained by an electrical connection between a heat sink and a groundingpattern inside an insulating layer of the thermally conductive board andto provide a method of manufacturing the same. In addition, it also isan object of the present invention to provide a power module allowingits size to be reduced and its density increased using the thermallyconductive board.

In order to achieve the above-mentioned object, a thermally conductivecircuit board of the present invention includes an electricallyinsulating layer, an electrically conductive heat sink, and a lead frameas a wiring pattern. The electrically insulating layer is formed of athermally conductive resin composition containing 70 to 95 wt. %inorganic filler and 5 to 30 wt. % resin composition containing at leastthermosetting resin. The lead frame and the electrically insulatinglayer are provided with their surfaces flush with each other. Agrounding pattern electrically connected to the heat sink inside theelectrically insulating layer is present in an arbitrary position in thesame plane as that in which the wiring pattern is formed.

A first method of manufacturing a thermally conductive circuit boardwith a grounding pattern connected to a heat sink according to thepresent invention includes:

(1) placing a metal pole in a desired place in an electricallyconductive heat sink; and

(2) superposing a lead frame as a wiring pattern, a sheet-like thermallyconductive resin composition made of 70 to 95 wt. % inorganic filler and5 to 30 wt. % resin composition containing at least uncuredthermosetting resin, and the heat sink sequentially with a portion ofthe metal pole protruding from the heat sink facing the sheet-likethermally conductive resin composition, and heating and compressingthem,

so that (A) the metal pole is connected to the lead frame and thesheet-like thermally conductive resin composition is allowed to fill upto a surface of the lead frame or (B) a surface of the metal pole isallowed to be flush with the surface of the lead frame to be a part ofthe wiring pattern and the sheet-like thermally conductive resincomposition is allowed to fill up to the surface of the lead frame, and

the thermosetting resin contained in the sheet-like thermally conductiveresin composition is cured.

A second method of manufacturing a thermally conductive circuit boardwith a grounding pattern connected to a heat sink according to thepresent invention includes:

(1) processing an electrically conductive heat sink by extruding itsdesired portion to form a protrusion; and

(2) superposing a lead frame as a wiring pattern, a sheet-like thermallyconductive resin composition made of 70 to 95 wt. % inorganic filler and5 to 30 wt. % resin composition containing at least uncuredthermosetting resin, and the heat sink sequentially with the protrusionof the heat sink facing the sheet-like thermally conductive resincomposition, and heating and compressing them,

so that (A) the protrusion is connected to the lead frame and thesheet-like thermally conductive resin composition is allowed to fill upto a surface of the lead frame or (B) a surface of the protrusion isallowed to be flush with the surface of the lead frame to be a part ofthe wiring pattern and the sheet-like thermally conductive resincomposition is allowed to fill up to the surface of the lead frame, and

the thermosetting resin contained in the sheet-like thermally conductiveresin composition is cured.

A third method of manufacturing a thermally conductive circuit boardwith a grounding pattern connected to a heat sink according to thepresent invention includes:

(1) placing a metal pole in a desired place in a lead frame as a wiringpattern; and

(2) superposing the lead frame, a sheet-like thermally conductive resincomposition made of 70 to 95 wt. % inorganic filler and 5 to 30 wt. %resin composition containing at least uncured thermosetting resin, andan electrically conductive heat sink sequentially with a portion of themetal pole protruding from the lead frame facing the sheet-likethermally conductive resin composition, and heating and compressingthem, so that the metal pole is connected to the heat sink, thesheet-like thermally conductive resin composition is allowed to fill upto a surface of the lead frame, and the thermosetting resin contained inthe sheet-like thermally conductive resin composition is cured.

A fourth method of manufacturing a thermally conductive circuit boardwith a grounding pattern connected to a heat sink according to thepresent invention includes:

(1) inserting a metal pole into a desired place in a sheet-likethermally conductive resin composition made of 70 to 95 wt. % inorganicfiller and 5 to 30 wt. % resin composition containing at least uncuredthermosetting resin; and

(2) superposing a lead frame as a wiring pattern, the sheet-likethermally conductive resin composition, and an electrically conductiveheat sink sequentially, and heating and compressing them, so that themetal pole is connected to both the heat sink and the lead frame, thesheet-like thermally conductive resin composition is allowed to fill upto a surface of the lead frame, and the thermosetting resin contained inthe sheet-like thermally conductive resin composition is cured.

A fifth method of manufacturing a thermally conductive circuit boardwith a grounding pattern connected to a heat sink according to thepresent invention includes:

(1) preparing an uncured electrically conductive resin compositioncontaining at least electrically conductive metal powder andthermosetting resin;

(2) making a hole in a desired place in a sheet-like thermallyconductive resin composition made of 70 to 95 wt. % inorganic filler and5 to 30 wt. % resin composition containing at least uncuredthermosetting resin and filling the hole with the electricallyconductive resin composition; and

(3) superposing a lead frame as a wiring pattern, the sheet-likethermally conductive resin composition, and an electrically conductiveheat sink sequentially, and heating and compressing them, so that theelectrically conductive resin composition is connected to both the heatsink and the lead frame, the sheet-like thermally conductive resincomposition is allowed to fill up to a surface of the lead frame, andthe thermosetting resins contained in the sheet-like thermallyconductive resin composition and the electrically conductive resincomposition are cured.

A power module of the present invention includes a thermally conductivecircuit board, an active device and a passive device for power, and acontrol circuit. The thermally conductive circuit board includes anelectrically insulating layer, an electrically conductive heat sink, anda lead frame as a wiring pattern. The electrically insulating layer isformed of a thermally conductive resin composition containing 70 to 95wt. % inorganic filler and 5 to 30 wt. % resin composition containing atleast thermosetting resin. The lead frame and the electricallyinsulating layer are provided with their surfaces flush with each other.A grounding pattern electrically connected to the heat sink inside theelectrically insulating layer is present in an arbitrary position in thesame plane as that in which the wiring pattern is formed. The active andpassive devices for power are mounted on the thermally conductivecircuit board. Furthermore, the control circuit is connected thereto forenabling the power module to be controlled.

According to the present invention, a ground connection portion can beprovided in a desired position in a wiring pattern of a board includinga sheet-like thermally conductive resin composition containing aninorganic filler added to a thermosetting resin composition at a highratio, an electrically conductive heat sink, and a lead frame as thewiring pattern. Therefore, the degree of freedom in design is improvedand a grounding pattern can be provided during the production of theboard. Furthermore, since the ground connection outside the board is notrequired, the size of the board can be reduced. As a result, thegrounding pattern and the heat sink can be connected electrically witheach other in an arbitrary place inside the insulating layer of thethermally conductive circuit board. Consequently, it is possible toprovide a thermally conductive circuit board with high heat dissipation,high conductivity, and high ground-connection reliability, a method ofmanufacturing the same, and a power module allowing its size to bereduced and its density to be increased.

In addition, according to the methods of manufacturing a thermallyconductive circuit board of the present invention, a highly reliablethermally conductive board can be manufactured by a simple method.

Furthermore, according to the power module with a thermally conductivecircuit board of the present invention, since ground connection outsidethe board is not required, the degree of freedom in designing the moduleis improved and the size of the module can be reduced.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view showing a thermally conductive boardaccording to an embodiment of the present invention.

FIGS. 2A to 2C show sectional views illustrating respective steps in amethod of manufacturing a thermally conductive board according to anembodiment of the present invention.

FIG. 3 is a sectional view showing a thermally conductive board with ametal pole surface used as a grounding pattern according to anotherembodiment of the present invention.

FIGS. 4A to 4C show sectional views illustrating respective steps in amethod of manufacturing a thermally conductive board with a protrudingportion provided in a metal heat sink to establish a ground connection,according to still another embodiment of the present invention.

FIG. 5 is a sectional view showing a thermally conductive board with aprotruding portion of a heat sink serving as a grounding patternaccording to another embodiment of the present invention.

FIGS. 6A to 6C show sectional views illustrating respective steps in amethod of manufacturing a thermally conductive board with a metal poleplaced in a lead frame, according to another embodiment of the presentinvention.

FIGS. 7A to 7C show sectional views illustrating respective steps in amethod of manufacturing a thermally conductive board with a metal poleinserted in a sheet-like thermally conductive resin composition,according to still another embodiment of the present invention.

FIGS. 8A to 8C show sectional views illustrating respective steps in amethod of fitting a metal pole into a heat sink according to anotherembodiment of the present invention.

FIGS. 9A to 9E show sectional views illustrating respective steps in amethod of manufacturing a thermally conductive board including a metalpole provided with two step protrusions to establish a ground connectionaccording to another embodiment of the present invention.

FIGS. 10A to 10C show sectional views illustrating respective steps in amethod of manufacturing a thermally conductive board with a metal polehaving a conoidal tip according to still another embodiment of thepresent invention.

FIGS. 11A to 11D show sectional views illustrating respective steps in amethod of manufacturing a thermally conductive board with anelectrically conductive resin composition used to establish a groundconnection according to yet another embodiment of the present invention.

FIG. 12 is a sectional view showing a power module according to anembodiment of the present invention.

FIG. 13 is a sectional view showing a power module according to anotherembodiment of the present invention.

FIG. 14 is a sectional view showing a power module according to stillanother embodiment of the present invention.

FIGS. 15A and 15B show sectional views illustrating respective steps ina method of manufacturing a conventional thermally conductive board.

FIG. 16 is a sectional view showing a ground connection method used in aconventional thermally conductive board.

DETAILED DESCRIPTION OF THE INVENTION

According to the above-mentioned thermally conductive circuit board ofthe present invention, the grounding pattern can be placed in anarbitrary position in the same plane as that in which the wiring patternis formed. Therefore, the degree of freedom in pattern design isimproved. In addition, the connection inside the insulating layerenables the size of the board to be reduced. Moreover, since thegrounding pattern is provided in the same plane as that in which theother pattern is formed, the thermally conductive circuit board isadvantageous in component mounting and connection.

According to a preferred example in which the grounding pattern isformed of the lead frame and the grounding pattern and the heat sink areconnected electrically with each other inside the electricallyinsulating layer, the lead frame is used as the grounding pattern.Therefore, a higher degree of freedom in shape is obtained, andcomponent mounting on and connection to the grounding pattern arefacilitated.

According to a preferred example in which the grounding pattern and theheat sink are connected electrically with each other through a metalpole embedded in the insulating layer, the use of the metal pole allowsa connection portion to have a lower resistance and thus the connectionreliability is improved. In addition, a high position accuracy in thethermally conductive circuit board allows the connection reliability tobe improved.

According to a preferred example in which the grounding pattern and theheat sink are connected electrically with each other through a curedmaterial of an electrically conductive resin composition containing atleast electrically conductive metal powder and thermosetting resinprovided in the electrically insulating layer, an electrical connectioncan be obtained with high accuracy.

According to a preferred example in which the grounding pattern isformed of a part of the heat sink processed to form a protrusion with aflat top, the part of the heat sink can be used as the grounding patternwithout requiring any further processing. Therefore, a high connectionreliability can be obtained.

According to a preferred example in which the grounding pattern isformed of a top of the metal pole connected to the heat sink, the metalpole for a connection can be used as the grounding pattern withoutrequiring any further processing.

Preferably, the metal pole is fitted in at least one of the lead frameand the heat sink. Furthermore, it also is preferable that at least oneend of the metal pole has a two step protrusion.

One end of the metal pole may have a conoidal shape, and the conoidalend of the metal pole may be stuck in one of the heat sink and the leadframe. In this specification, the “conoidal shape” includes a “pyramidalshape”.

According to the first method of the present invention, an insulatorwith a high inorganic filler ratio and excellent thermal conductivitycan be handled easily. In addition, a part of the lead frame as thewiring pattern can be used as a grounding pattern in forming the board.

In another method of the present invention, the above-mentioned process(2) is replaced by a step of superposing a lead frame as a wiringpattern, the sheet-like thermally conductive resin composition, and theheat sink sequentially with a portion of the metal pole protruding fromthe heat sink facing the sheet-like thermally conductive resincomposition, and heating and compressing them, so that a surface of themetal pole is made flush with the surface of the lead frame to be a partof the wiring pattern, the sheet-like thermally conductive resincomposition is allowed to fill up to the surface of the lead frame, andthe thermosetting resin contained in the sheet-like thermally conductiveresin composition is cured. According to this method, the metal pole canbe used as a connection material and also can be used as the groundingpattern, so that it is no longer necessary to prepare the groundingpattern separately.

In each method of manufacturing a thermally conductive circuit boardmentioned above, the process (1) may be replaced by a step of processingan electrically conductive heat sink by extruding its desired portion ina protrusion form. Furthermore, in the process (2), the metal pole maybe replaced by a protrusion. In this case, it is no longer necessary toprepare the metal pole. Since a part of the heat sink can be used as thegrounding pattern without requiring any further processing or can beconnected directly to the lead frame, the connection reliability isimproved.

In the first to fourth methods of the present invention, it ispreferable that a hole with a slightly smaller diameter than that of themetal pole is formed in a desired place in at least one of the heat sinkand the lead frame and the metal pole is fitted into the hole. Thisimproves the connection reliability.

In each method of manufacturing a thermally conductive circuit boarddescribed above, preferably, an end to be connected to the heat sink orthe lead frame of ends of the metal pole or the protrusion has aconoidal shape, and in the process (2), the end with a conoidal shape ofthe metal pole or the protrusion bites into and is connected to the heatsink or the lead frame. This improves the connection reliability and theposition accuracy.

Similarly in the fifth method of the present invention, the groundingpattern can be provided in an arbitrary place in a wiring pattern.

In the power module of the present invention, active and passive devicesfor power are mounted on any one of the thermally conductive circuitboards described above and a control circuit is connected thereto. Thisconfiguration can provide a small power module with a high energydensity, high heat dissipation, and high degree of freedom in design.

The power module of the present invention is characterized in thatcomponents are mounted directly on a grounding pattern that is notincluded in the wiring pattern of the thermally conductive circuitboard.

According to the present invention, since a ground connection portioncan be provided in a desired position in the wiring pattern, the degreeof freedom in design is improved. In addition, since a pattern forgrounding can be provided during the production of the board, thepresent invention is advantageous in terms of the number of steps.Furthermore, it is not necessary to provide a connection portion outsidethe board, and therefore the size of the module can be reduced.

The term “power module” used in this specification refers to those witha plurality of semiconductors connected depending on their applicationsand contained in one package. Examples of the power module include aninsulated gate bipolar transistor (IGBT) module, an intelligent powermodule (IPM), and a DC—DC converter module.

In the present invention, a sheet-like thermally conductive resincomposition is used that can be prepared by processing a mixtureincluding a high ratio of inorganic filler added to an uncuredthermosetting resin into a sheet form. This thermally conductive resincomposition has a high inorganic filler blending ratio and therefore hashigh thermal conductivity. Consequently, when the thermally conductiveresin composition is used as an insulating material in the board, theinsulation distance can be increased while high heat dissipation ismaintained and thus a board can be manufactured that is excellent innoiseproof and insulation properties, which is an advantage. Inaddition, the thermally conductive resin composition is characterized inthat the cured thermally conductive resin composition has a thermalexpansion coefficient close to those of a semiconductor and metal as thewiring pattern and has high reliability with respect to heat history.Since the thermally conductive resin composition has flexibility atordinary temperatures, it can be handled easily and can be processed bypunching or the like. In addition, since it has excellent resinflowability, not only can the thermally conductive resin composition becombined easily with the wiring pattern and the heat sink to form onebody, but also the side faces of a thick wiring pattern can beresin-molded. Thus, a board can be manufactured that is excellent ininsulating property and in flatness.

Next, embodiments of the present invention are based on the following:the above-mentioned thermally conductive resin composition is heated andcompressed together with a lead frame as a wiring pattern and anelectrically conductive heat sink, so that they are combined to form onebody; the thermally conductive resin composition is allowed to fillopenings of the lead frame to form a flush surface; the thermosettingresin contained in the thermally conductive resin composition is curedwhile the heat sink is allowed to adhere to the thermally conductiveresin composition; and thus a rigid board (thermally conductive board)with excellent heat dissipation is produced.

In a first embodiment of the present invention, during the formation ofthe thermally conductive board described above, a protrusion such as ametal pole or the like is provided in an arbitrary place in the wiringpattern or the heat sink in advance, the protrusion is placed andsuperposed to face the thermally conductive resin composition, and thewhole is heated and compressed, so that a ground connection isestablished between the heat sink and a part of the wiring patternduring the formation of the board.

In a second embodiment of the present invention, during the formation ofthe thermally conductive board described above, a protrusion such as ametal pole or the like is provided in an arbitrary place in the heatsink in advance, the protrusion is placed and superposed to face thethermally conductive resin composition, and the whole is heated andcompressed, so that the protrusion is formed as a grounding pattern atthe same time the board is formed.

In a third embodiment of the present invention, during the formation ofthe thermally conductive board described above, a metal pole or anelectrically conductive resin composition is inserted into an arbitraryplace in a thermally conductive resin composition preformed in a sheetshape so as to go through the sheet-like thermally conductive resincomposition, this is superposed with a lead frame as a wiring patternand a heat sink, they are heated and compressed to be combined to formone body, and a ground connection is established between a part of thewiring pattern and the heat sink.

A fourth embodiment of the present invention is directed to a powermodule in which active and passive devices for power are mounted on athermally conductive circuit board formed in one of the embodimentsdescribed above and particularly components are mounted directly on thegrounding pattern.

In this specification, a thermally conductive mixture includes athermosetting resin composition and an inorganic filler, and thethermosetting resin composition includes at least thermosetting resin,and a curing agent, a curing accelerator, and various additives. Thethermally conductive mixture has a thermal conductivity of 2 to 15 W/mK.With respect to the maximum thermal conductivity of the inorganicfiller, for example, Al₂O₃ and AlN have maximum thermal conductivitiesof 30 W/mK and 200 W/mK, respectively. The minimum amount of thethermosetting resin in the thermosetting resin composition is 30 wt. %.In addition, the heat sink has a maximum resistivity of 30 μΩ·cm (=0.3μΩ·m), and the electrically conductive resin composition also has amaximum resistivity of 30 μΩ·cm (=0.3 μΩ·m) after cured.

Methods of manufacturing a thermally conductive board and a power moduleaccording to the embodiments of the present invention are described withreference to the drawings as follows.

FIG. 1 is a sectional view of a thermally conductive board provided witha grounding pattern according to an example of the present invention. Inthe drawing, numeral 11 indicates a lead frame as a wiring pattern,numeral 12 an insulating layer made of a mixture of 70 to 95 wt. %inorganic filler and 5 to 30 wt. % resin composition containing at leastthermosetting resin, numeral 13 an electrically conductive heat sink,numeral 14 a metal pole, and numeral 15 a grounding pattern. Thegrounding pattern 15 and the heat sink 13 are connected with each otherinside the insulating layer 12 through the metal pole 14.

FIGS. 2A to 2C show sectional views illustrating respective steps in amethod of manufacturing a thermally conductive board according to anexample of the present invention. In FIG. 2A, numeral 11 indicates alead frame, numeral 16 a sheet-like thermally conductive resincomposition obtained by formation of a film with a mixture of 70 to 95wt. % inorganic filler and 5 to 30 wt. % resin composition containing atleast uncured thermosetting resin, numeral 13 an electrically conductiveheat sink, and numeral 14 a metal pole. As shown in FIG. 2A, the metalpole 14 is inserted in the heat sink 13 and protrudes from the surfaceof the heat sink 13. The above-mentioned members are superposed as shownin FIG. 2B and then are heated and compressed. Thus, as shown in FIG.2C, the thermally conductive resin composition 16 fills openings of thelead frame 11 to provide a flush surface. At the same time, thethermosetting resin contained in the thermally conductive resincomposition 16 is cured, and thus an insulating layer 12 is obtained andis combined with the lead frame 11 and the heat sink 13 to form onebody. In this state, the metal pole 14 is connected to the groundingpattern 15 in the lead frame 11 as a wiring pattern. Consequently, athermally conductive board with a grounding pattern is completed.

The “lead frame” in this case refers to those formed to be one body witha metal plate processed to have a wiring pattern shape and respectivewiring patterns connected with each other using an outer frame as a partof the metal plate. The lead frame is not particularly limited as longas it is made of a low-electric-resistance metal. For example, copper,iron, aluminum, nickel, or alloys thereof can be used for the leadframe. In addition, for instance, a method employing chemical etching orpunching with a die can be used as the pattern formation method.Preferably, the surface of the lead frame is plated for the purpose ofimproving antioxidation and solder wettability properties. For example,nickel, tin, solder, gold, silver, palladium, chromium, or alloyscontaining them as a main component can be used as a plating material.

Preferably, the inorganic filler is contained in the thermallyconductive resin composition in a ratio in the range of 70 to 95 wt. %.When the ratio of the inorganic filler is lower than the range, thethermal conductivity of the thermally conductive resin componentdecreases and this causes excessively high heat resistance of the board.Consequently, it becomes difficult to dissipate heat generated from anactive device mounted on the board to the outside. Thus, the reliabilityof the device deteriorates. In addition, the thermal expansioncoefficient of the thermally conductive resin composition increases andthis increases the difference in thermal expansion coefficient betweenthe thermally conductive resin composition and components such as asemiconductor or metal portion such as a wiring pattern. As a result,the reliability deteriorates. On the contrary, higher ratios of theinorganic filler than the range cause the deteriorations in insulationproperty and in adhesiveness between the thermally conductive resincomposition and each of the lead frame and the heat sink.

Preferable inorganic fillers include those containing, as a maincomponent, powder of at least one selected from Al₂O₃, AlN, SiC, Si₃N₄,MgO, SiO₂, and BN, since they are excellent in thermal conductivity andelectrical insulation property and enable a board with high heatdissipation to be manufactured. Particularly, when Al₂O₃ or SiO₂ isused, it can be mixed with the resin component easily. The use of AlNallows the thermally conductive board to have a particularly high heatdissipation. Furthermore, it is preferable that the inorganic filler hasa grain size in the range of 0.1 to 100 μm, since inorganic fillershaving larger or smaller grain sizes than those in the above-mentionedrange cause deterioration in the filling property of the filler and inthe heat dissipation of the board. Particularly, in order to obtain anexcellent filling property of the inorganic filler, it is preferable toblend powders with various grain sizes in the above-mentioned range toprovide a high-density filling configuration.

Preferably, the main component of the thermosetting resin contained inthe thermosetting resin composition is at least one selected from epoxyresin, phenolic resin, and cyanate resin, since these resins areexcellent in heat resistance, mechanical strength, and electricalinsulation. Particularly, it is preferable that the resin compositioncontains brominated polyfunctional epoxy resin as the main component,and further contains bisphenol A-type novolac resin as a curing agentand imidazoles as a curing accelerator. This is because thisconfiguration is excellent not only in heat resistance but also in flameretardancy.

It is preferable that at least one selected from a coupling agent, adispersant, a coloring agent, and a mold releasing agent further isadded to the resin composition. The coupling agent is preferable in thatit improves the bond strength between the thermosetting resin and eachof the inorganic filler, the lead frame, and the heat sink, and thewithstand voltage of the board. For instance, an epoxysilane-,aminosilane-, or titanate-based coupling agent can be used as thecoupling agent. The dispersant is preferable in that it improves thedispersibility of the inorganic filler contained in the thermallyconductive resin component and homogenizes the inorganic filler. Forexample, phosphate ester can be used as the dispersant. The coloringagent is preferable in that it colors the thermally conductive resincomposition to allow its heat radiation to be improved. For instance,carbon can be used as the coloring agent.

The film formation method for forming the thermally conductive resincomposition in a sheet form is not particularly limited. A doctor blademethod, a coating method, an extrusion process, or the like can be usedas the film formation method. A solvent may be mixed with the thermallyconductive resin composition to adjust the viscosity thereof and thenmay be dried at a lower temperature than a curing temperature of thethermosetting resin contained in the thermally conductive resincomposition, and thus the thermally conductive resin composition may beformed in a sheet form. In this case, preferably, the doctor blademethod is used, since it permits easy film formation. As the solvent,for example, methyl ethyl ketone (MEK), toluene, or isopropanol can beused.

For instance, aluminum, copper, iron, nickel, or alloys thereof can beused for the electrically conductive heat sink. In addition, any metalscan be used for the metal pole as long as they are excellent inelectrical conductivity. For example, aluminum, copper, iron, nickel, oralloys thereof can be used for the metal pole. Particularly, it ispreferable that the metal pole is made of the same material as that ofthe electrically conductive heat sink, since in this case, the heat sinkand the metal pole have an identical thermal expansion coefficient andtherefore a poor connection does not tend to occur easily between theheat sink and the metal pole due to temperature changes.

FIG. 3 is a sectional view of a thermally conductive board with agrounding pattern according to another example of the present invention.In the figure, numeral 11 indicates a lead frame as a wiring pattern,numeral 12 an insulating layer formed of a mixture of 70 to 95 wt. %inorganic filler and 5 to 30 wt. % resin composition containing at leastthermosetting resin, numeral 13 an electrically conductive heat sink,and numeral 14 a metal pole. This thermally conductive board can beproduced by the same method as shown in FIGS. 2A to 2C. However, themetal pole 14 is not in contact with the lead frame 11 and the top ofthe metal pole 14 is used as a grounding pattern 15 without requiringany further processing.

The same materials as those described with reference to FIG. 2 can beused for the metal pole according to the present embodiment. Preferably,however, a coating film is formed on the top of the metal pole by aplating method or the like to provide excellent solder wettability sincethe top is used as the grounding pattern. For instance, solder, tin,silver, palladium, or alloys thereof can be used for the coating film.

FIGS. 4A to 4C are sectional views showing respective steps in a methodof manufacturing a thermally conductive board according to anotherexample of the present invention. In FIG. 4A, numeral 11 indicates alead frame, numeral 16 a sheet-like thermally conductive resincomposition identical with that shown in FIG. 2A, and numeral 13 anelectrically conductive heat sink. As shown in FIG. 4A, a part of theheat sink 13 is extruded to be processed into a protrusion shape andthus is formed as a protrusion 17. These members are superposed as shownin FIG. 4B and then are heated and compressed. Thus, as shown in FIG.4C, the thermally conductive resin composition 16 fills openings of thelead frame 11 to provide a flush surface. At the same time, thethermosetting resin contained in the thermally conductive resincomposition 16 is cured, and thus an insulating layer 12 is obtained andis combined with the lead frame 11 and the heat sink 13 to form onebody. In this state, the protrusion 17 formed in the heat sink 13 isconnected to a grounding pattern 15 in the lead frame 11 as a wiringpattern. Consequently, a thermally conductive board with a groundingpattern is completed.

For example, a punch process, press working, or the like can be used asthe method of processing the protrusion.

FIG. 5 is a sectional view of a thermally conductive board with agrounding pattern according to still another example of the presentinvention. In the figure, numeral 11 indicates a lead frame as a wiringpattern, numeral 12 an insulating layer formed of a mixture of 70 to 95wt. % inorganic filler and 5 to 30 wt. % resin composition containing atleast thermosetting resin, numeral 13 an electrically conductive heatsink, and numeral 17 a protrusion formed in the heat sink 13. Thisthermally conductive board can be produced by the same method as shownin FIGS. 4A to 4C. However, the protrusion 17 is not in contact with thelead frame 11 and the top of the protrusion 17 is used as a groundingpattern 15 without requiring any further processing.

Preferably, a metal coating film with excellent solder wettability isformed on the top of the protrusion 17 by a plating method or the likeas in the embodiment shown in FIG. 3 since the top of the protrusion 17according to the present embodiment is used as the grounding pattern.

FIGS. 6A to 6C are sectional views showing respective steps in a methodof manufacturing a thermally conductive board according to yet anotherexample of the present invention. As shown in FIG. 6A, a metal pole 14is inserted in a certain pattern of a lead frame 11 as a wiring pattern.Next, as shown in FIG. 6B, the lead frame 11 with the metal pole 14 issuperposed with a sheet-like thermally conductive resin composition 16and an electrically conductive heat sink 13 prepared by the same methodsas those used for the composition and the heat sink shown in FIGS. 2A to2C. Then, they are heated and compressed. Thus, as shown in FIG. 6C, thethermally conductive resin composition 16 fills openings of the leadframe 11 to provide a flush surface. At the same time, the thermosettingresin contained in the thermally conductive resin composition 16 iscured, and thus an insulating layer 12 is obtained and is combined withthe lead frame 11 and the heat sink 13 to form one body. In this state,the metal pole 14 provided in the lead frame 11 is connected to the heatsink 13. A part of the lead frame 11 as a wiring pattern serves as agrounding pattern 15. Thus, a thermally conductive board with agrounding pattern is completed.

FIGS. 7A to 7C are sectional views showing respective steps in a methodof manufacturing a thermally conductive board according to anotherexample of the present invention. As shown in FIG. 7A, a metal pole 14is embedded in a certain portion of a sheet-like thermally conductiveresin composition 16 produced by the same method shown in FIG. 2A. Next,as shown in FIG. 7B, the thermally conductive resin composition 16 withthe metal pole 14 embedded therein is superposed with a lead frame 11 asa wiring pattern and an electrically conductive heat sink 13, and thenthey are heated and compressed. Thus, as shown in FIG. 7C, the thermallyconductive resin composition 16 fills openings of the lead frame 11 toprovide a flush surface. At the same time, the thermosetting resincontained in the thermally conductive resin composition 16 is cured, andthus an insulating layer 12 is obtained and is combined with the leadframe 11 and the heat sink 13 to form one body. In this state, the metalpole 14 provided in the thermally conductive resin composition 16 isconnected to the heat sink 13 and the lead frame 11. A part of the leadframe 11 as the wiring pattern serves as a grounding pattern 15. Thus, athermally conductive board with a grounding pattern is completed.

FIGS. 8A to 8C are sectional views showing respective steps in a methodof fitting a metal pole into a heat sink according to another example ofthe present invention. As shown in FIG. 8A, a through hole 18 isprovided in an electrically conductive heat sink 13 in an arbitraryplace. Next, as shown in FIG. 8B, a metal pole 14 with a slightly largerdiameter than that of the through hole 18 is positioned on the throughhole 18 and then is fitted into it. Thus, as shown in FIG. 8C, a heatsink with a metal pole 19 is produced. Then, a thermally conductiveboard provided with a grounding pattern is completed by the same methodas shown in FIGS. 2A to 2C.

For example, a hand press or eccentric press can be used as the fittingmethod. According to this method, a simple and convenient board can beproduced that is excellent in connection reliability. Furthermore, ametal pole can be provided in a lead frame by being fitted thereinto bythe same method as shown in FIGS. 8A to 8C. In this case, a thermallyconductive board is completed by the method as shown in FIGS. 6A to 6C.

FIGS. 9A to 9E are sectional views showing respective steps in a methodof manufacturing a thermally conductive board according to still anotherexample of the present invention. As shown in FIG. 9A, a metal pole 20with two step protrusions is prepared. Next, as shown in FIG. 9B, thismetal pole 20 is positioned on a through hole 18 of an electricallyconductive heat sink 13 and then is fitted thereinto. Thus, a heat sink13 with a metal pole as shown in FIG. 9C is produced. Then, a sheet-likethermally conductive resin composition 16 produced by the same method asin the above-mentioned embodiments and a lead frame 11 as a wiringpattern provided with a through hole 18 are prepared and are superposedon the heat sink 13 with a metal pole. Then, they are heated andcompressed. Thus, as shown in FIG. 9E, the thermally conductive resincomposition 16 fills openings of the lead frame 11 to provide a flushsurface. At the same time, thermosetting resin contained in thethermally conductive resin composition 16 is cured, and thus aninsulating layer 12 is obtained and is combined with the lead frame 11and the heat sink 13 to form one body. In this state, a protrusion ofthe metal pole 20 provided in the heat sink 13 is connected to thethrough hole 18 provided in the lead frame 11. A part of the lead frame11 as the wiring pattern serves as a grounding pattern 15. Thus, athermally conductive board with a grounding pattern is completed.

FIGS. 9A to 9E show an embodiment in which one protrusion of the metalpole 20 is connected to the lead frame 11. However, the protrusion ofthe metal pole 20 may not be connected to the lead frame 11 but may beexposed at the board surface and the protrusion itself may be used asthe grounding pattern 15. In this case, a constant creeping distance canbe maintained to keep the desired electrical insulation between thegrounding pattern 15 and the wiring pattern therearound. In addition,since the larger-diameter portion of the metal pole 20 serves as aspacer, the distance between the wiring pattern and the heat sink can beset to be shorter. Thus, this case is preferable, since a higher-densityboard can be obtained.

In addition, FIGS. 9A to 9E show the metal pole provided with the twostep protrusions at both ends. However, the metal pole may be providedwith a two step protrusion only at one end. In this case, the protrusionof the metal pole is fitted into a through hole provided in one of theheat sink and the lead frame. Then, a thermally conductive board with agrounding pattern can be produced by the same methods as shown in FIGS.2A to 2C, FIG. 3, and FIGS. 6A to 6C.

The descriptions with reference to FIGS. 8A to 8C and FIGS. 9A to 9Ewere directed to embodiments in which a through hole is provided in theheat sink or the lead frame. However, it may not be always necessary forthe hole into which the metal pole is to be fitted to go through theheat sink or the lead frame as long as the metal pole can be fitted intothe hole.

FIGS. 10A to 10C are sectional views showing respective steps in amethod of manufacturing a thermally conductive board according to yetanother example of the present invention. As shown in FIG. 10A, a metalpole 14 with one end 21 having a conoidal shape is provided in anelectrically conductive heat sink 13 by the same method as describedwith reference to FIGS. 8A to 8C. Next, a lead frame 11 as a wiringpattern and a sheet-like thermally conductive resin composition 16produced by the same method as in each embodiment described above areprepared and are superposed on the heat sink 13 as shown in FIG. 10B.Then, they are heated and compressed. Thus, thermosetting resincontained in the thermally conductive resin composition 16 is cured, andthus an insulating layer 12 is formed and is combined with the leadframe 11 and the heat sink 13 to form one body as shown in FIG. 10C. Atthe same time, the conoidal end 21 of the metal pole 14 provided in theheat sink 13 sticks into the lead frame 11 to be connected thereto. Apart of the lead frame 11 as the wiring pattern serves as a groundingpattern 15. Thus, a thermally conductive board with a grounding patternis completed.

FIGS. 11A to 11D are sectional views showing respective steps in amethod of manufacturing a thermally conductive board according toanother example of the present invention. As shown in FIG. 11A, athrough hole 18 is formed in an arbitrary place in a sheet-likethermally conductive resin composition 16 produced by the same method asin each embodiment described above. Next, as shown in FIG. 11B, thethrough hole 18 is filled with an electrically conductive resincomposition 22. Then, as shown in FIG. 11C, the thermally conductiveresin composition 16, a lead frame 11 as a wiring pattern, and anelectrically conductive heat sink 13 are superposed and then are heatedand compressed. Thus, thermosetting resin contained in the thermallyconductive resin composition 16 is cured, and thus an insulating layer12 is formed and is combined with the lead frame 11 and the heat sink 13to form one body as shown in FIG. 11D. At the same time, theelectrically conductive resin composition 22 in the through hole 18provided in the thermally conductive resin composition 16 also is curedand electrically connects the lead frame 11 and the heat sink 13. A partof the lead frame 11 as the wiring pattern serves as a grounding pattern15. Thus, a thermally conductive board with a grounding pattern iscompleted.

Materials including electrically conductive metal powder mixed withthermosetting resin can be used as the electrically conductive resincomposition. Preferably, the metal powder is made of at least one metalselected from a group consisting of gold, silver, copper, nickel,palladium, tin, and solder, and alloys thereof. This is because suchmetal powder is excellent in conductivity and has high connectionreliability. For instance, an epoxy resin can be used as thethermosetting resin. In addition, it is preferable that the electricallyconductive resin composition is mixed in a paste form, since this allowsthe through hole in the thermally conductive resin component to befilled easily. For example, screen printing can be used as the fillingmethod.

The above-mentioned composition of the electrically conductive resin andfilling method using the same are preferable in that not only when thewiring pattern and the heat sink are made of the same material but alsowhen they are made of different materials, the difference in thermalexpansion between the respective different materials is compensated andthus a highly reliable electrical connection can be obtained.

In each embodiment described above, when the lead frame, thermallyconductive resin composition, and heat sink are superposed and then areheated and compressed to form a board, a portion of the thermallyconductive resin composition corresponding to the position where themetal pole or the protrusion is to be provided may be removed inadvance. This is because, in this case, the metal pole or protrusion ispassed through the thermally conductive resin composition easily andthus, the possibility of deteriorating the conductivity due to presenceof the thermally conductive resin composition is reduced. Punchingmethods with a punch or mold can be used as a method of removing thepart of the thermally conductive resin composition.

Furthermore, in each embodiment described above, a plurality ofgrounding patterns may be provided in a board as required by a circuit.A plurality of methods according to the embodiments described above maybe used as the grounding method.

FIG. 12 is a sectional view of a power module including a thermallyconductive board according to one of the examples of the presentinvention. In FIG. 12, the board portion has the same configuration asshown in FIG. 1. Numeral 11 indicates a lead frame as a wiring pattern,numeral 12 an insulating layer formed of a mixture of 70 to 95 wt. %inorganic filler and 5 to 30 wt. % resin composition containing at leastthermosetting resin, numeral 13 an electrically conductive heat sink,numeral 14 a metal pole, and numeral 15 a grounding pattern. Thegrounding pattern 15 and the heat sink 13 are connected inside theinsulating layer 12 through the metal pole 14. Parts of the lead frame11 as the wiring pattern are bent to be orthogonal to the board to formexternal terminals. An active device 23 and a passive device 24 aremounted on the wiring pattern 11 of this board, and a control circuitboard 27 placed thereabove is connected to the lead frame 11 as thewiring pattern and as the external terminals. The control circuit board27 includes circuit components 26 mounted on a prepared circuit board25. The circuit portion is sealed with resin and further steps ofattaching a case and the like are carried out as required, and thus, apower module is completed. Since such steps can be carried out by knownmethods, they are not described in detail.

The method of mounting respective components may be selected suitablydepending on the types of components. In the case of a surface mountingcomponent as shown in FIG. 13, soldering can be employed. In FIG. 13,numerals 30 and 31 indicate a surface mounting component and a solderingportion, respectively. In the case of a semiconductor bare chip as shownin FIG. 14, a wire bonding method can be employed. In FIG. 14, numerals32 and 33 indicate a semiconductor device and a metal wire,respectively. In addition, the connection between wiring patterns alsomay be carried out with a lead wire or a metal bridge in mounting thecomponents.

A general printed circuit board can be used as the circuit board 25 fora control circuit. For example, a glass epoxy board or a phenol boardcan be used. Furthermore, for example, a method of inserting a boardwith a control circuit mounted thereon into the external terminals ofthe thermally conductive board as shown in FIG. 12 can be used as themethod of mounting the control circuit on the thermally conductiveboard.

In the above-mentioned power module, preferably, the components aremounted on the grounding pattern 15, since this allows a smallhigh-density power module to be provided.

Moreover, in FIG. 12, the same configuration as shown in FIG. 1 is usedfor the thermally conductive board. However, the configuration is notlimited to this and each thermally conductive substrate shown in FIGS. 3to 11D can be used.

EXAMPLES

A thermally conductive board of the present invention, a method ofmanufacturing the same, and a power module using the same are describedfurther in detail by means of specific examples as follows.

Example 1

In order to produce a sheet-like thermally conductive resin compositionused in the present invention, an inorganic filler and a thermosettingresin composition were mixed and processed in a slurry form. Thecomposition of the thermally conductive resin composition thus obtainedis described as follows.

-   (1) Inorganic filler: 89 wt. % Al₂O₃ (AS-40, with a mean grain size    of 12 μm, manufactured by Showa Denko Co., Ltd.)-   (2) Thermosetting resin: 10 wt. % brominated polyfunctional epoxy    resin (NVR-1010, containing a curing agent, manufactured by Japan    REC Co., Ltd.)-   (3) Other additives: 0.5 wt. % curing accelerator (imidazole,    manufactured by Japan REC Co., Ltd.), 0.4 wt. % carbon black    (manufactured by Toyo-Carbon Co., Ltd.), and 0.55 wt. % coupling    agent (Preneact KR-46B, manufactured by Ajinomoto Co., Inc.)

Methyl ethyl ketone (MEK) was added to those materials as a solvent andfurther alumina balls were added thereto. They were mixed using a ballmill at 800 rpm for 40 hours. The addition of MEK allows the viscosityof the mixture to decrease and the mixture to be processed in a slurryform. However, MEK is allowed to evaporate in the later drying step andtherefore is not mentioned in the discussion of the blended composition.

This slurry was applied to a releasing film of polyethyleneterephthalate (PET) whose surface had been subjected to a mold-releasetreatment, to form a film by a doctor blade method. Afterward, the filmwas dried at 90° C. and thereby the solvent evaporated. Thus, asheet-like thermally conductive resin composition 16 shown in FIG. 2Awas produced. The sheet-like thermally conductive resin composition 16thus obtained had a thickness of 1.2 mm.

Next, a 0.5-mm thick copper sheet (manufactured by Kobe Steel, Ltd.) wasetched by a known method and thus a circuit pattern was formed. Thecopper sheet with the circuit pattern was nickel-plated and thus a leadframe 11 as shown in FIG. 2A was prepared. Furthermore, a 1-mm thickaluminum sheet 13 was prepared. A through hole with a diameter of 1.45mm was formed in a part of the aluminum sheet 13. Then, an aluminumcolumn with a diameter of 1.50 mm and a length of 2.0 mm was fitted intothe through hole as shown in FIGS. 8A to 8C. Thus, a heat sink 13 with ametal pole 14 as shown in FIG. 2A was prepared.

The members thus prepared were superposed as shown in FIG. 2B and thenwere heated and compressed at a temperature of 150° C. under a pressureof 3 MPa for 10 minutes. As a result, the metal pole 14 provided in thealuminum heat sink 13 was connected to the lead frame 11 through thesheet-like thermally conductive resin composition 16. At the same time,openings of the lead frame were filled with the thermally conductiveresin composition by the compression, and thus a flush surface wasobtained. In addition, the thermosetting resin was cured by the heatingand thus an insulating layer was formed. Then, the insulating layer,lead frame, and heat sink were combined to form one body. Consequently,a rigid board (with a thickness of 2.5 mm) as shown in FIG. 2C wascompleted.

The resistance values of the heat sink 13 and the grounding pattern 15in the board were measured and as a result, were not higher than 0.1Ω.Furthermore, in order to evaluate connections in the board, the boardwas passed through a reflow device whose peak temperature was 240° C.ten times and then the resistance values were measured again. As themeasurement result, no variation was found. Even after thermal cycletests at −40 to 125° C. repeated 500 times, no variation in theresistance values was found. This proved the high connection reliabilityof the grounding pattern.

Example 2

The following description is directed to an example of a thermallyconductive board in which the surface of a metal pole inserted in a heatsink is used as a grounding pattern.

A sheet-like thermally conductive resin composition and a lead frameproduced by the same methods as in Example 1 were prepared. In addition,a 3-mm thick aluminum sheet was prepared and a recessed portion with a 2mm×2 mm square shape and a depth of 1.5 mm was provided in a part of thealuminum sheet. An aluminum metal pole having a length of 3 mm with atwo step protrusion at its one end was prepared. The protrusion had alength of 1.5 mm. The flat surface of the metal pole on which theprotrusion was not formed was solder-plated. Next, the two stepprotrusion (2.05 mm×2.05 mm square) of the metal pole was fitted intothe recessed portion of the aluminum sheet. Thus, a heat sink with ametal pole as shown in FIG. 2A was produced.

The heat sink with a metal pole, the lead frame, and the sheet-likethermally conductive resin composition were superposed as shown in FIG.2B, and were heated and compressed at a temperature of 150° C. under apressure of 3 MPa for 10 minutes. Consequently, the metal pole providedin the aluminum heat sink was passed through the sheet-like thermallyconductive resin composition and the surfaces of the metal pole and thelead frame were made flush. At the same time, by the compression,openings of the lead frame were filled with the thermally conductiveresin composition and thus a flush surface was provided. Furthermore,the thermosetting resin was cured by the heating and thus an insulatinglayer was formed. This insulating layer, lead frame, and heat sink werecombined to form one body. Consequently, a rigid board (with a thicknessof 4.5 mm) as shown in FIG. 3 was completed.

The resistance values of the heat sink and the grounding pattern in theboard were measured and as a result, were not higher than 0.1Ω.Furthermore, in order to evaluate connections in the board, the boardwas passed through a reflow device whose peak temperature was 240° C.ten times and then the resistance values were measured again. As themeasurement result, no variation was found. Even after thermal cycletests at −40 to 125° C. repeated 500 times, no variation in theresistance values was found. This proved the high connection reliabilityof the grounding pattern. In addition, an eutectic solder paste wasprinted on the grounding pattern, and then the board was passed throughthe same reflow device as described above. Thus, the solder wettabilitywas evaluated and excellent wettability and no abnormality were found.

Example 3

The following description is directed to an example of a thermallyconductive board in which a desired portion of a heat sink was extrudedto be processed in a protrusion form and a grounding pattern isconnected to the heat sink using the protrusion.

A sheet-like thermally conductive resin composition was produced by thesame method as in Example 1. The composition of this thermallyconductive resin composition is described as follows.

-   (1) Inorganic filler: 90 wt. % Al₂O₃ (AS-40, with a mean grain size    of 12 μm, manufactured by Showa Denko Co., Ltd.)-   (2) Thermosetting resin: 9 wt. % cyanate ester resin (“AroCy M30”,    manufactured by Asahi Ciba Co., Ltd.)-   (3) Other additives: 0.4 wt. % carbon black (manufactured by    Toyo-Carbon Co., Ltd.), 0.2 wt. % dispersant (“PLYSURF, A-208F”,    manufactured by Daiichi Kogyo Seiyaku Co., Ltd.), and 0.4 wt. %    silane coupling agent (A-187, manufactured by Nippon Unicar Co.,    Ltd.)

Next, a circuit pattern was formed by punching in a 42-alloy (an alloywith a composition of about 42 wt % nickel-iron) sheet with a thicknessof 0.5 mm and was nickel-plated. Thus, a lead frame was prepared. Then,a 0.5-mm thick copper sheet was prepared and a protrusion having a topwith a diameter of 2 mm was formed in a predetermined position as shownin FIG. 4A by pressing.

The members thus prepared were superposed and were heated and compressedat a temperature of 170° C. under a pressure of 4 MPa for 50 minutes. Asa result, the protrusion provided in the heat sink passed through thesheet-like thermally conductive resin composition and was connected tothe lead frame. At the same time, the thermally conductive resincomposition filled openings of the lead frame to provide a flushsurface. In addition, the thermosetting resin was cured by the heatingand thus, an insulating layer was formed. The insulating layer, leadframe, and heat sink were combined to form one body and thus a rigidboard (with a thickness of 1.5 mm) as shown in FIG. 4C was completed.

The resistance values of the heat sink and the grounding pattern in theboard were measured and as a result, were not higher than 0.1Ω.Furthermore, in order to evaluate connections in the board, the boardwas passed through a reflow device whose peak temperature was 240° C.ten times and then the resistance values were measured again. As themeasurement result, no variation was found. Even after thermal cycletests at −40 to 125° C. repeated 500 times, no variation in theresistance values was found. This proved the high connection reliabilityof the grounding pattern.

Example 4

The following description is directed to an example of a thermallyconductive board in which a metal pole is embedded in a sheet-likethermally conductive resin composition and is connected to a groundingpattern and a heat sink.

In order to produce the sheet-like thermally conductive resincomposition, an inorganic filler and thermosetting resin were kneaded,and thus a clay-like thermally conductive resin composition wasobtained. The composition of the thermally conductive resin compositionis described as follows.

-   (1) Inorganic filler: 35 wt. % AlN (SCAN70, manufactured by the Dow    Chemical Company) and 55 wt. % Al₂O₃ (AS-40, manufactured by Showa    Denko Co., Ltd.)-   (2) Thermosetting resin: 9.5 wt. % epoxy resin (XNR5002,    manufactured by Nagase-Ciba, Ltd.)-   (3) Other additives: 0.3 wt. % silane-based coupling agent (A-187,    manufactured by Nippon Unicar Co., Ltd.) and 0.2 wt. % carbon black    (manufactured by Toyo-Carbon Co., Ltd.).

These materials were blended and kneaded with a planetary mixer. Thus, ahomogeneous composition was obtained and then was processed in a 1.2-mmthick sheet form with a extrusion machine. Consequently, a sheet-likethermally conductive resin composition was produced.

Next, an aluminum metal pole with a diameter of 2 mm and a length of 1.2mm was produced in which both end portions with a length of 0.1 mm wereprocessed to have a conoidal shape. The metal pole was inserted into thethermally conductive resin composition with a hole punched in a desiredposition in a predetermined form as shown in FIG. 7A. A lead frameproduced by the same method as in Example 1 and a 1.5-mm thick aluminumsheet were prepared.

The members thus prepared were superposed as shown in FIG. 7B and wereheated and compressed at a temperature of 170° C. under a pressure of 8MPa for 60 minutes. Consequently, the metal pole inserted in thesheet-like thermally conductive resin composition was connected to thelead frame and heat sink. At the same time, the thermally conductiveresin composition filled openings of the lead frame to provide a flushsurface. In addition, the thermosetting resin was cured by the heatingand thus, an insulating layer was formed. The insulating layer, leadframe, and heat sink were combined to form one body and thus a rigidboard (with a thickness of 3.0 mm) as shown in FIG. 7C was completed.

The resistance values of the heat sink and the grounding pattern in theboard were measured and as a result, were not higher than 0.1Ω.Furthermore, in order to evaluate connections in the board, the boardwas passed through a reflow device whose peak temperature was 240° C.ten times and then the resistance values were measured again. As themeasurement result, no variation was found. Even after thermal cycletests at −40 to 125° C. repeated 500 times, no variation in theresistance values was found. This proved the high connection reliabilityof the grounding pattern. In addition, the portion of the board wherethe metal pole was embedded was cut and the cutting plane was observed.In this portion, the conoidal portions of the metal pole bit into thelead frame and the heat sink.

Example 5

The following description is directed to an example of a thermallyconductive board in which a grounding pattern and a heat sink areconnected with each other through an electrically conductive resincomposition.

A sheet-like thermally conductive resin composition with a thickness ofabout 0.7 mm was produced by the same method as in Example 1. A throughhole with a diameter of 0.8 mm as shown in FIG. 11A was provided in thesheet-like thermally conductive resin composition by punching. As shownin FIG. 11B, this through hole was filled with an electricallyconductive resin composition for via-hole filling by screen printing.The electrically conductive resin composition for via-hole filling wasprepared by kneading, using three rollers, 85 wt. % spherical coppermetallic particles, 3 wt. % bisphenol A-type epoxy resin (EPICOAT 828,manufactured by Yuka Shell Epoxy Co., Ltd.) and 9 wt. % glycidyl esterepoxy resin (YD-171, manufactured by Toto Kasei) as resin compositions,and 3 wt. % amine adduct curing agent (M-24, manufactured by AjinomotoCo., Inc.) as a curing agent. Furthermore, a 0.4-mm thick copper sheetwas etched to form a circuit pattern and then was tin-plated. Thus, alead frame was prepared. In addition, a heat sink formed of a 0.8-mmthick copper sheet was prepared.

These materials were superposed as shown in FIG. 11C and were heated andcompressed at a temperature of 180° C. under a pressure of 5 MPa for 10minutes. Consequently, the thermally conductive resin composition filledopenings of the lead frame to provide a flush surface. In addition, thethermosetting resin was cured by heating and thus, an insulating layerwas formed. The insulating layer, lead frame, and heat sink werecombined to form one body and thus, a rigid board (with a thickness of1.7 mm) as shown in FIG. 11D was completed. In this case, theelectrically conductive resin composition also was cured at the sametime the sheet-like thermally conductive resin composition was cured,and was connected to the lead frame and heat sink.

The resistance values of the heat sink and the grounding pattern in theboard were measured and as a result, were not higher than 0.1Ω.Furthermore, in order to evaluate connections in the board, the boardwas passed through a reflow device whose peak temperature was 240° C.ten times and then the resistance values were measured again. As themeasurement result, no variation was found. Even after thermal cycletests at −40 to 125° C. repeated 500 times, no variation in theresistance values was found. This proved the high connection reliabilityof the grounding pattern.

Example 6

The following description is directed to an example of a power moduleproduced using a thermally conductive board according to the presentinvention.

A power semiconductor device of a TO-220 package (manufactured byMatsushita Electric Industrial Co., Ltd.) and various passive devicessuch as a capacitor, transformer, choke, resistance or the like weremounted on a wiring pattern of the thermally conductive board producedin Example 2 and were soldered using a reflow device. Afterward, a frameportion of a lead frame was cut and its ends were bent to be orthogonalto the board, thus forming lead-out electrodes. Consequently, a powercircuit board with a power circuit mounted thereon was completed.

Further, various ICs and passive components were surface-mounted on aglass epoxy board having four wiring layers and thus a control circuitboard with a driving circuit and a protection circuit formed therein wasproduced. In this control circuit board, through holes were formed inplaces corresponding to the positions of the lead-out electrodes of thepower circuit board.

Then, the control circuit board was inserted into the power circuitboard and the lead-out electrodes of the power circuit board wereallowed to pass through the through holes in the control circuit board.Afterward, the lead-out electrodes and connection portions of thecontrol circuit board were soldered. Then, a circuit portion was moldedwith silicone resin (manufactured by Shinetsu Silicone Co., Ltd.) andthen was inserted into a case. Thus, a DC—DC converter module as shownin FIG. 12 was completed.

A similar power circuit was formed using a thermally conductive boardwith no grounding pattern. Since the circuit design requires a part ofthe lead frame terminal and the heat sink to be connected with eachother, the area of the thermally conductive substrate increased by about1.1 times. Furthermore, it also is necessary to connect a wire for theconnection between the grounding pattern and the heat sink. Therefore,the numbers of components and places to be soldered increased. Thisproves that the thermally conductive substrate connecting the groundingpattern and heat sink inside the insulating layer has an effect on thereductions in processing steps and in size of the module.

The invention may be embodied in other forms without departing from thespirit or essential characteristics thereof. The embodiments disclosedin this application are to be considered in all respects as illustrativeand not limiting. The scope of the invention is indicated by theappended claims rather than by the foregoing description, and allchanges which come within the meaning and range of equivalency of theclaims are intended to be embraced therein.

1. A method of manufacturing a thermally conductive circuit board with agrounding pattern connected to a heat sink, the method comprising: (1)placing a metal pole in a desired place in an electrically conductiveheat sink; and (2) superposing a lead frame as a wiring pattern, asheet-like thermally conductive resin composition made of 70 to 95 wt. %inorganic filler and 5 to 30 wt. % resin composition containing at leastuncured thermosetting resin, and the electrically conductive heat sinksequentially with a portion of the metal pole protruding from theelectrically conductive heat sink facing the sheet-like thermallyconductive resin composition, and heating and compressing them, so that(A) the metal pole is connected to the lead frame and the sheet-likethermally conductive resin composition is allowed to fill up to asurface of the lead frame or (B) a surface of the metal pole is allowedto be flush with the surface of the lead frame to be a part of thewiring pattern and the sheet-like thermally conductive resin compositionis allowed to fill up to the surface of the lead frame, and thethermosetting resin contained in the sheet-like thermally conductiveresin composition is cured.
 2. The method of manufacturing a thermallyconductive circuit board according to claim 1, further comprisingforming a hole in a desired place in at least one of the electricallyconductive heat sink and the lead frame, the hole having a shapecorresponding to a shape of the metal pole, and fitting the metal poleinto the hole.
 3. The method of manufacturing a thermally conductivecircuit board according to claim 1, wherein an end of the metal pole tobe connected to the electrically conductive heat sink or the lead framehas a conoidal shape, and in the process (2), the end with the conoidalshape of the metal pole bites into and is connected to the electricallyconductive heat sink or the lead frame.